Proton and Carbon Linacs for Hadron Therapy

Beams of 200 MeV protons and 400 MeV/u fully stripped carbon ions are used for the treatment of solid tumours seated at a maximum depth of 27 cm. More than 100’000 patients have been treated with proton beams and more than 10’000 with carbon ions. Very low proton currents of the order of 1 nA are enough to deliver the typical dose of 2 Gy/l in one minute. In the case of carbon ions the currents are of the order of 0.1-0.2 nA. For this reason 3 GHz linacs are well suited in spite of the small apertures and low duty cycle. The main advantage of linacs, pulsing at 200-400 Hz, is that the output energy can be continuously varied pulse-by-pulse and in 2-3 min a moving tumour target can be covered about 10 times by deposing the dose in many thousands of ‘spots’. High frequency hadron therapy linacs have been studied in the last 20 years and are now being built as hearts of proton therapy centres, while carbon ion linacs are still in the designing stage. At present the main challenges are the reduction of the footprint of compact ‘single-room’ proton machines and the power efficiency of dual proton and carbon ions ‘multi-room’ facilities. INTRODUCTION In developed countries more than 2’000 patients every 1 million inhabitants are subject to radiation therapy every year [1]. In more than 95% of cases, the treatment of the tumour disease consists in irradiation with X-rays. Hadron therapy has developed in the last 60 years as an advanced technique in radiation therapy that allows a not invasive and precise irradiation of solid tumours with the advantage of sparing the surrounding healthy tissues. The proposal, published in 1946 by ‘Bob’ Wilson [2], was based on the presence of the Bragg peak in the depth-dose profile of charged hadrons. Figure 1: Comparison of depth-dose profile for X-rays (black) and charged hadrons (blue). The energy released by a beam of mono-energetic charged hadrons is concentrated at the end of the range in matter. Since by changing the beam energy one can adjust the depth of the Bragg peak, the overlapping of many Bragg peaks produces a flat dose distribution in the tumour region, as seen in Fig. 1. The same figure shows that most of the energy deposited by an X-ray beam is outside of the tumour target so that, even with many crossed beams, in an X-ray-treatment, healthy normal tissues are in a ‘bath’ of radiation, while the total energy delivered to normal tissues with charged hadrons is typically a factor 3-4 smaller than with X-rays. The use of linacs for hadron therapy has been proposed in the late eighties. Since 1993, TERA Foundation, in parallel with the study of a synchrotron based dual facility for the Italian hadrontherapy project concluded with the construction of CNAO in Pavia [3] initiated the study of high frequency linacs for hadron therapy applications. In 2009 a review of the subject was published by Reviews of Accelerator Science and Technology [4].